The invention is based on a switched-mode power supply with a storage capacitor, a transformer with a primary winding and a secondary winding, and with a switching transistor, which is connected in series with the primary winding. Switched-mode power supplies of this type are used in particular as flyback converters in entertainment electronics equipment, for example in television sets and video recorders.
The switching transistor used in a switched-mode power supply of this type is used in a switching mode with the shortest possible switching and blocking times, in order to minimize the losses in the switching transistor. The resulting current interruptions at the blocking time, the high di/dt caused thereby and the inductances of the transformer at the beginning of the blocking phase of the switched-mode power supply produce a high-voltage peak over the switching transistor which significantly exceeds the steady-state voltage value defined by the storage capacitor. If the switched-mode power supply is operated on the 230-volt AC system, voltage peaks up to 1000 V may thus occur, representing a risk to the switching transistor, or the latter must be designed accordingly for this voltage range.
For this purpose, it is known for a damping network, also referred to as a snubber network, to be provided, which at least partially suppresses these voltage peaks. A frequently used damping network of this type is, for example, known from DE 40 29 221 A1. Here, a network, comprising a capacitor connected in series with a diode, with which a resistor is connected in parallel, is disposed in parallel with the primary winding of the transformer. By means of this damping network, the voltage peak occurring on the switching transistor is at least partially suppressed by the capacitor, or is stored and subsequently fed off via the diode to the storage capacitor, or is consumed via the resistor when the capacitor is discharged.
In smaller switched-mode power supplies, it is also known for only one capacitor, connected in parallel with the primary winding, to be used as the damping network. However, this circuit variant produces a higher current loading of the switching transistor at the activation time. Further circuit variants for damping voltage peaks are known, for example, from EPA-A-0 279 335.
The object of the invention is to provide a damping network for a switched-mode power supply of the type mentioned at the beginning, which has only a few components and produces low losses in the switched-mode power supply.
The switched-mode power supply according to the invention has a storage capacitor, a transformer with a primary winding and at least one secondary winding, and also a switching transistor, which is connected in series with the primary winding. The primary winding is subdivided into sub-windings with at least one tap, and, as the damping network, a capacitor is in each case disposed in parallel with a sub-winding, preferably with each sub-winding. By means of this measure, the sub-windings of a primary winding are individually damped, and not the entire primary winding as such.
The numbers of windings of the sub-windings and the capacitances of the capacitors disposed in parallel with the sub-windings are advantageously selected in such a way that the oscillations produced when the switching transistor becomes non-conductive have different resonant frequencies and thereby at least partially cancel each other. This results in an effective damping of the voltage across the switching transistor when it becomes non-conductive. However, since the capacitors are connected in series here, the resulting total capacitance is small, so that the discharge current concerned is comparatively low at the time of conduction of the switching transistor.
A xe2x80x9cchamber transformerxe2x80x9d in particular can be used as the transformer, of the type known, for example, from EP-A-0 071 008, in which the primary winding is subdivided into sub-windings which are disposed in individual chambers of a coil shell, whereby a tap can be fed off to each sub-winding with no significant increase in cost.